The present invention relates in general to motor drive circuits and, more particularly, to circuits for driving electric motors with variable pulse width drive signals.
Electric motors currently are being used in a wide variety of consumer and industrial applications. For example, automobile manufacturers are using electric motors to replace combustion engines in motor vehicles. Such motors often are induction type electric motors in which the field windings consist of three coils connected to a common central node. The other ends of the coils are driven by three phase voltages mutually separated in phase by 120.degree.. The phase voltages produce sinusoidal currents in the field coils for driving the motor.
Since automobile electric motors operate from a battery, it is important that they operate efficiently in order to minimize battery consumption and recharging. Consequently, most motor drive circuits use voltage pulses to drive the coils rather than phase voltage waveforms. The drive pulses are generated in an ordered sequence with variable pulse widths to produce an envelope whose shape approximates that of the desired phase voltages.
A number of schemes are in use for generating such motor drive pulse sequences. For example, with standard pulse width modulation (PWM), pulses are computed for each coil independently and the coils are driven with sinusoidal phase voltages. However, PWM fixes the central node at a constant potential, which reduces the maximum power available to drive the motor. Another scheme, space vector modulation (SVM), treats the coil drive pulses as a unit, which increases maximum power by allowing the central node to vary in potential. Such prior art schemes often can be selected to optimize the performance of a motor under one set of conditions. However, the performance deteriorates when the conditions change.
For example, it is desirable to use electric motors in automobiles to extend operating time by recharging the battery when the vehicle is decelerating using kinetic energy stored in the vehicle's mass. Hence, during motoring, the electric motor delivers energy to propel the vehicle, whereas during regeneration, the motor functions as a generator to transfer energy back to the battery. Prior art schemes which minimize power consumption during motoring suffer from low efficiency during regeneration, and vice versa.
Hence, there is a need for a circuit and method of adapting a motor drive system to changing conditions to maintain a high overall efficiency.